Promote draw merge final composite task

docs/modernization/tasks.md

@@ -2051,6 +2051,34 @@ Validation:
 ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure
 ```
 
+### STR-047 - Extract Draw Merge Final Composite Gating
+
+Status: Ready
+Score: no score movement
+Debt: `DEBT-0036`
+Scope: `src/canvas.cpp`, `src/legacy_canvas_draw_merge_services.h`, `tests/paint_renderer/compositor_tests.cpp`
+
+Goal:
+
+Move the remaining `Canvas::draw_merge()` final composite gating into a
+retained helper so the plane-loop helper only dispatches plane setup and layer
+branches.
+
+Done Checks:
+
+- `execute_canvas_draw_merge_plane_iteration(...)` no longer owns the final
+  composite gating inline.
+- Regression coverage proves the helper preserves the `use_blend` final
+  composite decision.
+- `docs/modernization/debt.md` records the reduced draw-merge final-composite
+  surface.
+
+Validation:
+
+```powershell
+ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure
+```
+
 ### STR-010 - Extract Remaining Draw Merge Composite Orchestration
 
 ### STR-016 - Extract Draw Merge Layer Composite Execution